Model of heating load of anaerobic fermentation tank and test on its influencing factors of biogas plant

Abstract: Calculation accuracy of heating load in mixed anaerobic fermentation tank is related to design rationality, stability and economic operation of the whole system, so it is very important to master the heating load model and understand the influence of main factors on its characteristics. Aiming at the heat process of the whole mixed anaerobic fermentation tank of the actual biogas project in Shanghai, physical and mathematical model of heating load was established. Five indicators, i.e. monthly average load percentage, monthly heat dissipation rate of the envelope, monthly mean daily capacity of the pool, total heating load of pool capacity and designed heating load of pool, are used to analyze the size of heating load of the various components and the impact on the annual heat. The heating load of the biogas project mainly includes the feeding load and the heat dissipation of the envelope, in which the heating load of the feed liquid is related to the feeding amount, the feeding temperature and the temperature of the fermentation tank. The heat dissipation of the envelope is mainly related to the fermentation temperature, pool body structure materials, and so on. Therefore, it is necessary to study the relationship between the heating load characteristics and the fermentation temperature. This study takes a small fermentation tank with a volume of 15 m3 in Jiading Campus of Tongji University in Shanghai as the study object. The temperature of the fermentation tank is (25±1), (30±1) and (35±1) ℃, the total feeding amount is 750 kg per day, and the feeding time is between 10:00 and 10:30. It is concluded that the heat dissipation capacity of the envelope is larger than that of the feeding load, and the total heating load is closely related to the temperature of the fermentation tank, the ambient temperature and the temperature of the liquid. The effects of different fermentation temperatures and top film thicknesses of the insulation layer on the heating load show that the heating loads in Shanghai area under the fermentation temperature of (30±1) and (35±1) ℃ are respectively 1.54 and 1.94 times that under the fermentation temperature of (25+1) ℃. Compared with the fermentation temperature of 30 ℃, the total heating load under the fermentation temperature of 35 ℃ is increased by about 40%, while the heat is gotten more difficultly, and the heat pump unit COP (coefficient of performance) decreased by about 0.6 when the temperature of inlet water at source side is same, so the fermentation temperature with economic effects is determined as 30 ℃. At present, for large and medium-sized biogas project in China, the appropriate insulation measures are used in the bottom of pool and around the pool wall, but between the top of the membrane and the atmosphere the insulation measures are not taken, and the heat dissipation of the top of the pool is much larger than the pool and the bottom. The effects of different thicknesses of roof membrane’s rubber insulation layer of 0, 25, 50 and 75 mm on the heating load show that the heat dissipation load reduction rates of heat retaining structure are 67.99%, 16.49% and 7.28% respectively after the thickness of rubber insulation layer is increased by 25 mm in sequence, and the reduction rates of the total heating load are 48.02%, 7.17% and 2.85% respectively, so it is determined that the economic insulating layer thickness of top film is 50 mm in Shanghai area. The relative error is between 0.6% and 7.8% according to the comparison between the model and actual engineering test results of heating. The results can provide a reference for the calculation of the heating load of the biogas project and the thickness of the insulation layer.